The present invention relates generally to laser navigation systems and, more particularly, to an electrooptical system for sequentially switching a beam of light through a plurality of discrete beam positions.
A laser navigation system known in the art includes a laser whose output beam is directed via a beam pointing system to a plurality of azimuth locations relative to the motion vector of the vehicle to provide a time multiplexed transmitter beam distribution on the ground. The propagated beams are reflected from the ground, collected through the beam pointing system, and combined with a local oscillator signal in a detector. The detected Doppler signal may be processed to obtain information relating to, inter alia, velocity, angular position and range.
The overall system accuracy depends to a large extent on the quality of the Doppler velocity line-of-sight measurements which, in turn, rely on the accuracy of the beam pointing system in relation to the azimuth of the illumination locations and (more importantly) the angles of incidence, or depression angles, of the beams with the ground surface. Although beam pointing accuracy is of great importance, the repeatability of the beam pointing is of even greater significance.
One prior art system comprises a light source, a beam expander (telescope) and an optical device for directing the light from the source onto the optical axis of the beam expander. The beam expander is directed downward and at a predetermined oblique angle with respect to the ground surface. A motor drives the beam expander such that its optical axis sweeps a conical surface, and mechanical detents cause the beam expander to periodically stop and dwell at predetermined positions along its swept path. The device for directing light into the beam expander is also rotated intermittently to maintain the light beam at the input of the beam expander. The intermittently moving parts of this system are of relatively high inertia and are therefore difficult to maintain at close tolerances.
In a second system, similar to the one described above, the beam expander is fixed in position and an optically reflecting device is intermittently rotated to sweep the beam emerging from the beam expander to a plurality of locations on the ground surface. Although this second system provides a reduction in the intermittently driven mass, it is still very susceptible to relatively wide deviations of its azimuthal and depression angles in response to very slight deviations in the drive elements.
In another beam pointing apparatus, a light source and beam expander are pointed substantially downward, and a wedge-shaped prism is intermittently rotated normal to the beam axis. See, for example, U.S. Pat. No. 4,326,799, "Active-Passive Scanning System, " issued to W. H. Keene and A. V. Jelalian, on Apr. 27, 1982, and assigned to the same assignee as the present invention, for a description of a wedge-shaped prism used for beam steering. While useful in some applications, when used for intermittent scanning, the rotating wedge is susceptible to some of the same variations as the other prior art examples, resulting in wide deviations of the azimuthal and depression angles.
None of the aforementioned beam pointing apparatus is readily adaptable to providing the accuracy of Doppler velocity measurements required in many demanding navigational systems. Such systems require repeatability of measurements which can be obtained only by a beam pointing apparatus in which the azimuthal and depression angles of the ground illuminating beams deviate by no more than a few microradians. Such repeatability is virtually unattainable in an apparatus including a mechanically rotating beam pointing element. Control of the bearing runout tolerances and minimization of bearing noise to the extent necessary to prevent translation and unwanted rotation of the device would be costly and difficult to achieve and maintain.